1. Field of the Invention
The invention relates to a metal-ceramic filling for teeth to be cemented into a tooth cavity, comprising a ceramic filling that has a metal layer on the bottom and sides.
2. Description of Related Art
Metal ceramic fillings of this kind are already known from German Patent Disclosure Document 25 18 355 A1. It teaches a process for producing tooth crowns and dental fillings, in which a metal layer can be electrolytically deposited on a tooth model, and porcelain layers can be burned onto it. These metal layers may be of gold or a gold alloy with numerous other ingredients. The rim of the gold form may protrude and is polished, to achieve a tight fit in the gum region. An improvement in the dimensional accuracy and the fit at the periphery is attained with this process, as is an improvement in securing of the dental prostheses by means of the cement. The electrolytically deposited metal layer is smooth and ductile, and so it has limited adhesive strength and low flexural strength, and therefore only relatively small fillings can be made in this way and installed permanently.
It is also known from German Patent Disclosure Document 37 42 134 A1 to produce dental crowns in such a way that a ceramic layer having a thickness of from 200 to 300 .mu.m is fired on an electrolyte metal foundation. Once again, because of its inadequate strength, this metal layer is not suitable for fillings subject to major strain nor does it assure tooth retention over the long term.
It is further known from German Patent Disclosure Document 36 05 437 A1 to use cast metal with fired ceramic for dental prostheses; the cast metal is used only to produce the rims and for bridge parts subjected to mechanical strains.
Metal ceramic fillings, also known as metal ceramic inlays or on-lays are known from the brochure by Wieland Edelmetalle K.G. entitled "AURO, die echte Goldverblend-Krone [AURO: the genuine gold veneer crown] 1989, which are made by a ceramic firing technique. To this end, copies of the tooth stumps are made in the laboratory, and the duplicate stumps are provided with copper contact bars and coated with conductive paint. This duplicate stump is coated with a thin gold layer approximately 0.2 mm thick by electrolytic deposition. Its gold content is 99.0 to 99.1%. After this layer is finished with a gold bonder, the ceramic composition is fired on in layers. This process is known as the AGC or Auro-Galva-Crown technique.
It is also known for ceramic compositions to be fired in the laboratory on a nonmetal tooth stump, in order to make ceramic fillings, also known as ceramic inlays or on-lays, without a metal base.
Ceramic inlays/on-lays are also known which, after optical scanning of the prepared cavity in the patient's mouth and after computer evaluation, are milled out of a porcelain block by a robot. Since the occlusion is quite inaccurate in that case, it has to be corrected in the mouth. The very high investment cost for the equipment must be considered a further special disadvantage of this production technique.
Plastic inlays/on-lays are also known, which are prefabricated either directly by prefabrication and adaptation with pre-hardening in the patient's mouth and possible subsequent definitive hardening in the laboratory, but the direct polymerization in the patient's mouth can be very damaging to the patient's tooth pulp; such plastic inlays/on-lays are also made in an indirect process. In that case, the plastic is built up not in the patient's mouth but rather in the laboratory after a mold of the jaw has been made, on the tooth stump of the master model, and polymerized in the laboratory.
The pure ceramic and plastic inlays/on-lays are typically secured with glass ionomer cement, which is an aluminum silicate polyacrylic acid cement. Glass ionomer cements are preferred over phosphate cements, because they have lower solubility and greater adhesion. Since the edges of the fillings of these inlays/on-lays cannot be finished and sealed off, the cement must perform sealing between the tooth and the filling with respect to the environment of the mouth, and must additionally assure a maximum of adhesion, because as a result of contractions when the ceramic is fired or the plastic is polymerized and because of the relatively smooth internal surfaces, the forces of retention without this special glass ionomer cement are relatively weak. In plastic inlays/on-lays, there is also the possibility of curing the cement by light polymerization. Permanent sealing of the filling, however, cannot be achieved with any of the glass ionomer cements, because despite the relatively low solubility, over the years they loosen the body of the filling causing a lack of tightness between the tooth and the filling. Even if the fillings are firmly seated, a microscopic gap forms in some cases because of the relatively major retraction when these cements polymerize. The result is a recurrence of caries. Another disadvantage is that pulp damage can occur upon polymerization in situ.
In all the inlays/on-lays mentioned above, a parallel preparation of the tooth cavity walls is sought, in order to achieve a favorable distribution of chewing pressure on the ceramic or plastic filling and to achieve the maximum possible frictional force. Often, however, this is not possible without greatly weakening the tooth. Especially in the side regions of the tooth, where the chewing pressure is the highest, a filling with occlusal cusp protection, that is, an onlay, of pure ceramic or plastic is contraindicated because of the high risk of fracture of this material; recourse must therefore be made to a cosmetically unappealing metal cast filling with occlusal cusp protection (onlay), or to a metal ceramic crown, with the disadvantage of greater loss of tooth substance and possibly long-term periodontal damage.
In all the previously known inlays/on-lays, there is generally a high risk of fracture, because a stabilizing metal foundation is lacking. In the aforementioned AGC technique as well, there is no stable metal foundation, since the gold layer, with a thickness of 0.2 mm, is very thin, and is also very soft because of the gold content of 99.0-99.1%.
All ceramic fillings have the advantage that they are made of a material the color of teeth which are durable in color, biocompatible, and occlusion-stable, or in other words have high abrasion resistance, and are relatively plaque resistant. In addition, the temperature accumulation and temperature conductivity of the ceramic are largely equivalent to that of the natural tooth.
Although plastic inlays/on-lays are of material the color of teeth, they do not keep their color over the course of years. They also have less tissue compatibility and a lower abrasion resistance than ceramic or gold cast fillings, with cavities being particularly likely to occur in the side regions of the tooth, and less plaque resistance and a less favorable temperature accumulation and temperature conductivity than ceramic fillings.
Both ceramic and plastic fillings can be brought into direct contact with a metal alloy or with amalgam, because no galvanic voltage difference arises.
One advantage of the previously known metal ceramic fillings made by the Auro-Galva-Crown technique is as follows:
Once the ceramic composition has been fired onto the approximately 0.2 mm thick gold layer, a narrow gold rim, which is 0.2 mm thick and is soft because of the 99.0 to 99.1% gold content, remains, extending around the outside of the tooth; this can be pressed down. Since this gold rim is very thin and soft, it must remain out of occlusal contact. However, additional stability and masticatory pressure distribution is not provided by the thin, soft gold coating, but since additional sealing is assured by pressing down such rim, the gold rim makes it possible to dispense with the glass ionomer cement and instead to use a known fine-grained phosphate cement, for instance so-called Harvard cement, for fixation. Closing the gap with a rotating tool to bend the gold rim (burnishing) in a fully sealed and completely finished manner is not possible, however, because it is too soft.
Gold inlays/on-lays are also known as fillings, which are made by casting from a relatively soft gold alloy with approximately 85 to 90% gold. These gold fillings have high stability and edge and pressure resistance. They are chemically resistant and corrosion resistant, and can be sealed well by pressing down, burnishing and finishing the rims during the setting phase of the cement. In the ideal case, the finishing serves to press the still-soft cement all the way beneath the edges of the filling, and the gold rim rests directly on the tooth without any intervening layer of cement. In particular, the rim of the tooth cavity is broken when the cavity is prepared, producing a so-called spring rim on the filling, in order to compensate for metal contraction and to enable finishing and burnishing of the gold inlay rim.
When there is a narrow, thin filling rim that comes into occlusal contact and because of the use of relatively soft gold alloys and often merely because of the breakage of the rim of the cavity, it is possible that after a few years the filling loses its tightness from abrasion, which can cause fissure caries. For firmly seating the gold filling, a fine-grained phosphate cement suffices, e.g. the so-called Harvard cement, and there is no need to use the considerably more expensive glass ionomer cement, with its disadvantages of low pressure resistance, greater porosity, greater film thickness and greater dimensional change, which can cause fissures to form. The abrasion of the occlusal surface of the filling is less than that of plastic but higher than that of ceramic. In the highly polished state, a gold filling is also more tissue-compatible than plastic, but less tissue-compatible than ceramic. Moreover, because of the galvanic voltage difference it is not possible to place a gold filling in direct contact with another metal alloy or amalgam. Otherwise, pulp pain and a metal taste can occur.
Another advantage is that strict parallelism of the tooth cavity walls can be dispensed with, because of the great stability and edge and pressure resistance of the filling. The particular disadvantages are that the metal is cosmetically unsatisfactory and forms a major temperature accumulator and has high temperature conductivity, which can cause sensations of pain.